New simpler and more efficient way of naming script AVPs

In order to optimize and simplify the AVP naming, OpenSIPS trunk implements a new way of naming the AVPs.

The heavy way of using numerical ID and/or strings for names (via i: and s:), combined with static AVP aliasing (via core parameter) was dropped and replaced with a unified way of addressing AVPs.

With the new approach, AVPs are all the time addressed via string-based names (in script) - to be human friendly. These string names are automatically aliased to numerical names (internally), so the operations with AVPs will be as fast as possible.
Bottom line, the aliasing is done dynamically by OpenSIPS core, by mapping the string name from script to unique numerical ID in a dynamic and transparent way for the script writer.

Changes:

all AVPs are named via strings, so the "i:" and "s:" prefixes are deprecated

"avp_aliases" core param was dropped as useless.

Migrating is trivial: remove the "avp_aliases" parameter and all the "i:" and "s:" prefixes from AVP names.